1. Filed of the Invention
This invention relates to a projection cathode ray tube for producing an image to be projected onto a screen on an enlarged scale.
2. Description of the Related Art
FIG. 4 of the accompanying drawings shows a typical conventional video projector As shown in FIG. 4, the video projector comprises a projection cathode ray tube (hereinafter called "CRT") 1, light sources 1R, 1G, 1B for producing red (R), green (G) and blue (B) color images, respectively, and a series of projection lenses 2. Reference numeral 3 designates a screen 3 disposed in front of the CRT 1. An image is projected from the CRT 1 onto the screen 3 via the projection lenses 2 so as to produce a large color image.
FIG. 5 is a cross-sectional view of the CRT 1, which comprises a vacuum envelope 4, an electron gun 5 mounted at a neck portion of the vacuum envelope 4, a face plate 6 of the vacuum envelope 4, and a fluorescent layer 7 formed on an inner surface of the face plate 6. On the fluorescent layer 7, a vacuum evaporation aluminum film 8 is formed as a high voltage electrode and a deflecting plate. In the CRT 1, the electron gun 5 produces electron beams to excite the fluorescent layer 7, which then becomes luminous.
The conventional video projector is advantageous in producing a large colored image. But, there have hitherto been demand not only for a larger image but also for a high quality image, particularly for a brighter and finer image.
First, the technique for brightening the image will be described. To obtain a brighter image, there has been proposed a CRT, in which an optical interference filter 14 is disposed at a boundary 11 (FIG. 5) between the face plate 6 and the fluorescent layer 7 so as to increase the transmittivity of light beams which are nearly perpendicularly (about +30.degree.) incident on the face plate 6. This conventional technique is exemplified in Japanese Laid-open patent publication No. 207750/1983 discloses.
FIG. 8 shows a graph representing the spectroscopic transmittivity 12 of the interference filter 14 and the luminous spectrum of a green (G) fluorescent material. In FIG. 8, .theta. stands for the angle at which the light beams fall on the face plate 6 The light beams, which fall nearly perpendicularly on the interference filter 14, have the transmittivity of approximately 100%. The larger the incident angle, the lower the transmittivity is reduced sharply, and the more light beams will be reflected. The reflected light beams will be scattered and reflected again by the fluorescent layer 7 made of a high refractive index material. Then the more light beams will fall on the fluorescent layer 7 nearly perpendicularly.
FIG. 9(a) shows the distribution of beam intensity in the absence of any optical interference filter 14, while FIG. 8(b) shows the distribution of beam intensity in the presence of the interference filer 14.
The light beams as shown in FIG. 6 are emitted from the fluorescent material at the angle .alpha..sub.1 which is within +30 .degree. at the central areas of the CRT screen. As a result, use of the interference filter is very effective to produce a very bright image.
As shown in FIG. 8, an emission spectrum of the green fluorescent material includes not only essentially needed spectrum (a) but also needless spectra (b) to (d). With the interference filter 14, it is possible to minimize the needless spectra (c) and (d) as seen from FIG. 7, thus improving saturation of the green color.
However, the conventional video projector using the interference filter is disadvantageous in that the image is very bright at the central area of the CRT while it is very dark at the peripheral area of the CRT.
In FIG. 6, .theta..sub.2 is an angle at which a chief light beam 17 in an effective bundle of beams is incident onto the face plate 6, and is usually about +30.degree.. Also, since the angle .theta..sub.2 spreads over +.alpha..sub.2, the transmittivity will be considerably reduced so that the image will be very dark at the peripheral areas.
In order to overcome the problem, a proposal has been made to curve the inner surface of the face plate, i.e., the boundary 11, as shown in FIG. 7. The curved face plate serves to reduce the incident angle .theta..sub.2 of the chief beam at the peripheral areas of the CRT compared with that of the flat face plate shown in FIG. 5, and also increase the brightness of the image at the peripheral are of the CRT.
Further, if the face plate 6 is of such a shape so that the incident angle .theta..sub.2 is zero (0), namely, if the normal 16 at the boundary 11 at the peripheral area of the CRT has a radius of curvature passing through the incident pupil position 15 of the projection lens 2, the interference filter 14 can make an image very bright over the central and peripheral areas of the CRT. Practically, however, it is very difficult to manufacture a CRT having a very small radius of curvature.
To sum up, the conventional projection CRT using the interference filter can produce a very bright image at the central area of the CRT, while it produces a dark image at the peripheral areas of the CRT because the light beams are incident onto the interference filter at a large angle, which reduces the transmittivity.
A method to improve fineness of the image will now be described One of the main factors which hinder improving the fineness of the image is a glow observed around the luminous spot on the screen of the CRT, i.e., halo, which is caused by multiple reflection of the light beams (scattered beams).
The manner in which the halo is developed will be described with reference to FIG. 10 which shows the configuration of the fluorescent layer 7 of the CRT 1 illustrated in FIG. 5. Here the CRT 1 is a projection CRT (1G) producing green fluorescent beams In FIG. 10, reference numeral 70 designates fluorescent particles, which are usually 8 to 20 .mu.m in diameter. Light beams emitted from the aluminum film 8 excite fluorescent particles 70 to make them luminescent. Light beams passing through the boundary 11 between the face plate 6 and the fluorescent layer 7 reach the screen via the projection lens series not illustrated.
The light beams for example from the fluorescent particle 70 that is indicated with oblique lines are emitted in multiple directions. In the video projector, beams 9 directly passing through the boundary 11 are most intense and control the shape of a luminous spot. The intensity distribution of such light beams is indicated by a solid line in FIG. 11.
As shown in FIG. 10, light beams 10 are reflected by the boundary 11 and by the fluorescent layer, and some beams are reflected in multiple directions and then pass through the boundary 11.
These beams 10 cause halo. The beam intensity of the luminous spot is indicated by a dot-line arrow in FIG. 11. The larger the luminous spot, the more improving the fineness of fineness of the image will be hindered.
If an interference filter 14 is mounted as shown in FIG. 8, the transmittivity of light beams which are nearly perpendicularly incident onto the boundary 11 will be improved. But, the larger the incident angle .theta., the lower transmittivity and the larger the halo. In other words, the luminous spot in the presence of the interference filer 14 will become larger due to halo than in the absence of the filter 14.
As mentioned above, halo is caused by the light beams reflected on the boundary 11, and spreads over an area whose size depends mostly upon the size of vacuum gaps between the fluorescent particles 70 near the boundary 11. If the fluorescent particles are 8 to 12 .mu.m in diameter, the gaps between the particles 70 are 10 to 20 .mu.m. The diameter of the luminous spot affected by halo is about +10 to 20 .mu.m. If the CRT has a luminous spot diameter of 200 .mu.m, the spot diameter is increased by 10 to 20% due to halo.
To suppress halo, if the diameter of the fluorescent particles 70 is reduced to about 5 .mu.m, the gaps between the fluorescent particles 70 will be about 5 .mu.m near the boundary 11. This means that increase of the luminous spot diameter will be halved.
Small fluorescent particles, however, tend to be less luminescent and reduce the brightness of the image from the video projector. Even if the fluorescent layer 7 is thicker to increase the number of fluorescent particles, the transmittivity will be reduced in the thickness direction of the fluorescent layer 7, resulting in a reduced brightness of the image.